This project is a continuation of our study of the ultrastructure of the Limulus visual system, our long term goal being to correlate ultrastructure with the physiology of the compound eye. We are continuing our studies comparing the immature Limulus eye and the eyes of adults. Serial sections of ommatidia and plexus from embryos, young adults and adults will be examined to derive a developmental picture of the eye. We will also be examining ommatidia and related structures from single eyes, taking advantage of the fact that new units are added to the periphery of the eye until their adult number is attained. Sections taken at the periphery and progressively toward the older center of the eye will provide a developmental series from a single specimen and will help us test our proposed working hypothesis for the structure of the lateral plexus. A study of changes that occur in cone cells during maturation and growth will also be made in conjunction with the developmental series. We will also be continuing our effort to develop a reliable intracellular electron stain that can be used to identify individual branches within the complex neuropile. We intent to initiate new efforts to determine the effects of various drugs on the ultrastructure of the synapses and on the population of synaptic vesicles in the neuropile.